In recent years, in vehicles such as four-wheel-drive automobiles and two-wheel-drive automobiles (hereinafter such a vehicle will be referred to simply as an “automobile”), attempts have been made to employ an aluminum-alloy forged product in an internal combustion engine piston in order to attain high performance or to cope with environmental regulations. This is because, when such an aluminum-alloy forged product is employed, the weight of driving parts (e.g., a piston) for an internal combustion engine can be reduced, leading to reduction of a load upon operation of the internal combustion engine, enhancement of output, or reduction of fuel consumption. Conventionally, most internal combustion engine pistons have been produced from an aluminum-alloy cast product. However, in the case of such a cast product, difficulty is encountered in reducing internal defects generated during the course of casting, and excess material must be provided on the cast product so as to ensure safety design in terms of strength. Therefore, when such a cast product is employed in an internal combustion engine piston, reducing the weight of the piston is difficult. In view of the foregoing, attempts have been made to reduce the weight of such a piston by producing the piston from an aluminum-alloy forged product, in which generation of internal defects can be suppressed.
A conventional method for producing an aluminum-alloy forging material includes a step of preparing molten aluminum alloy by means of a typical smelting technique, a step of subjecting the molten aluminum alloy to any continuous casting technique, such as continuous casting, semi-continuous casting (DC casting) or hot top casting, to thereby produce an aluminum-alloy cast ingot and a step of subjecting the cast ingot to homogenization heat treatment to thereby homogenize aluminum alloy crystals. The thus produced aluminum-alloy forging material (cast ingot) is subjected to forging and then to T6 treatment to thereby produce an aluminum-alloy forged product.
JP-A 2002-294383 discloses a method for producing a 6000-series-alloy cast product, in which the homogenization treatment temperature is lowered or the homogenization treatment is omitted. However, this prior art does not describe high-temperature mechanical characteristics of the cast product.
In recent years, there has been increasing demand for an internal combustion engine of high efficiency and high output, and accordingly, parts employed in the engine have been further required to exhibit high-temperature mechanical strength.
An aluminum-alloy forged product produced through the aforementioned conventional method does not require provision of excess material since generation of internal defects in the forged product is suppressed. Therefore, when the forged product is employed in an internal combustion engine piston, the weight of the piston is reduced, as compared with the case where an aluminum-alloy cast product is employed. However, the forged product, in which crystallization products are formed into spherical aggregates, exhibits tensile strength at high temperatures of 300° C. or higher inferior to that of the aluminum-alloy cast product, in which crystallization product networks or acicular crystallization products formed during the course of casting remain. Therefore, in view of the fact that an aluminum-alloy forged product enables further reduction of the weight of an internal combustion engine piston, demand has arisen for a method for producing an aluminum-alloy shaped product exhibiting high-temperature mechanical strength superior to that of a conventional aluminum-alloy forged product.
In view of the foregoing, objects of the present invention are to provide a method for producing an aluminum-alloy shaped product that exhibits high-temperature mechanical strength superior to that of a conventional aluminum-alloy forged product, to provide an aluminum-alloy shaped product and to provide a production system for the shaped product.